Challenging programming topics

After a boring day spent programming Enterprise Java™, I like nothing better than to come home and program some more.

Currently I'm implementing the popular cryptographic algorithms in Go.

What programming topics do you use to challenge yourself?

I want to implement an amazing Shell framework .  Each command would be a all loaded by the shell and would provide standard functions and interfaces and shit . Like commands would register their options and he shell would process command line for them.  Also an interface to report progress of an operation back to he shell which can display this as it a
Sees fit .   So shells could be written different platforms and interface with existing command modules.  Also a shell could be graphical and disiplay an options dialog for the command line options, generated dynamically by querying the command's options.  A command could query if the shell Supports graphics and request a canvas to draw on instead of writing to Stdout